A conventional hologram recorder is disclosed in Patent Document 1 for example. In the hologram recorder, a laser beam from a light source is split by a half mirror in two directions. One of the split beams is modulated by a spatial light modulator which is provided by a liquid-crystal device. The spatial light modulator modulates the beam into a discrete beam which provides a digital (light and dark) pixel pattern in accordance with the information to be recorded. This beam is shone as a signal beam to a hologram recording medium. The other of the beams split by the half mirror is shone as a reference beam so that it will overlap with the signal beam on the hologram recording medium. Through this process, a hologram is recorded in the hologram recording medium, as an interference stripe pattern made by the signal beam and the reference beam.                Patent Document 1: JP-A-2002-216359        
In the above-described conventional hologram recorder, the spatial light modulator thins the signal beam discretely so that the beam makes a light-and-dark pixel pattern. The thinning of the beam, however, results in a large beam loss in the spatial light modulator.
Specifically, it is assumed, for example, that the amount of beam taken out for power monitoring is 15% of the initial amount of beam emitted from the light source; a ratio between the amount of signal beam and the amount of reference beam which reach the medium is 1:2; the transmissivity in each pixel of the spatial light modulator is 80%, and the spatial light modulator uses a 2-4 coding scheme (i.e. only one pixel out of each four-pixel block takes a bright state to express a two-bit code) for forming a light-and-dark pixel pattern, meaning that the ratio of light-transmitting area in an effective pixel area is 25%; and all the other losses in the amount of beam caused by the half mirror and so on are negligible. In this case, the amount of reference beam to reach the hologram recording medium is 24.3%, the amount of beam to enter the spatial light modulator is 60.7%, and the amount of signal beam to come out of the spatial light modulator and reach the medium is 12.1%. Thus, the beam loss at the spatial light modulator is 48.6% (=60.7%-12.1%), which means that approximately a half of the light emitted from the light source will be lost.